DiodesDiodes

What is a diode?What is a diode?

A diode is the A diode is the simplest form of simplest form of semi conductor semi conductor

devicedevice

What is a semi conductor?What is a semi conductor?

A semi conductor is defined A semi conductor is defined as a as a solidsolid whose whose

electrical conductivityelectrical conductivity can be can be controlled. controlled.

The Solid that is used in most The Solid that is used in most semi conductors is Silicon which semi conductors is Silicon which

is not to be mistaken with is not to be mistaken with siliconesilicone which is used for many thing such which is used for many thing such

as caulking around doorsas caulking around doors

SiliconSilicon is the most commercially is the most commercially important semiconductor used important semiconductor used

today.today.

Raw Silicon is a excellent Raw Silicon is a excellent insulator which would mean that insulator which would mean that

is a terrible choice for moving is a terrible choice for moving electrons. electrons.

The atomic stucture of Raw The atomic stucture of Raw Silicon is transformed by a Silicon is transformed by a

process called Doping.process called Doping.

Doping is the process of Doping is the process of introducing small amounts of introducing small amounts of

another compond to change the another compond to change the atomic structure of Siliconatomic structure of Silicon

Two of the most common Two of the most common compounds are compounds are BoronBoron and and

PhosphorusPhosphorus

PhosphorusPhosphorus is introduced to raw is introduced to raw Silicon to create a compound that Silicon to create a compound that has an excess of electrons which has an excess of electrons which

makes it a good conductor of makes it a good conductor of electricity with negative electricity with negative

properties. It is classified as a properties. It is classified as a N-type semiconductor. N-type semiconductor.

It is said that the electrons in It is said that the electrons in N-type Silicon are the Majority N-type Silicon are the Majority Carriers and the holes are the Carriers and the holes are the

Minority Carriers Minority Carriers

What are holesWhat are holes

In an abstract way holes are the In an abstract way holes are the space left when an electron moves. space left when an electron moves. A good way of understanding this A good way of understanding this is to think of removing an egg from is to think of removing an egg from an egg tray. When you remove the an egg tray. When you remove the egg a hole is left behind. egg a hole is left behind.

BoronBoron is introduced to raw is introduced to raw Silicon to create a compound that Silicon to create a compound that

has an excess of holes. This has an excess of holes. This makes it a good conductor of makes it a good conductor of

electricity with positive electricity with positive properties. It is classified as a properties. It is classified as a

P-type semiconductor.P-type semiconductor.

It is said that in P-type Silicon It is said that in P-type Silicon the holes are the Majority the holes are the Majority

Carriers and the electrons are Carriers and the electrons are the Minority Carriers the Minority Carriers

Materials with more of one type of Materials with more of one type of charge carrier than the other are charge carrier than the other are called extrinsic. The charge carrier called extrinsic. The charge carrier present in greater numbers is present in greater numbers is called the Majority Carrier. called the Majority Carrier. Materials with exactly the same Materials with exactly the same number of both types of carriers number of both types of carriers are called intrinsic.are called intrinsic.

In the simpliest terms a diode is a In the simpliest terms a diode is a device that is comprised of two device that is comprised of two

pieces of semi conductor material pieces of semi conductor material that are joined together. One that are joined together. One

piece of silicon would be N-type piece of silicon would be N-type silicon and the other would be P-silicon and the other would be P-

type silicontype silicon

The main purpose of a diode is to The main purpose of a diode is to allow electrons to flow in one allow electrons to flow in one

direction without resistance and direction without resistance and restrict electrons from flowing in restrict electrons from flowing in

the other direction.the other direction.

This is the schematic symbol for a This is the schematic symbol for a diode diode

For electricity to flow through a For electricity to flow through a diode it must be forward biased.diode it must be forward biased.

A forward biased diode will have A forward biased diode will have the Anode connected to the more the Anode connected to the more

positive side of the circuit and positive side of the circuit and the Cathode connected to the the Cathode connected to the

more negative side of the circuitmore negative side of the circuit

For a diode to work in a forward For a diode to work in a forward biased configuration it must be biased configuration it must be turned on. To turn on a diode turned on. To turn on a diode requires about .7 Volts to drop requires about .7 Volts to drop

over the diode.over the diode.

If a diode is placed in the circuit the If a diode is placed in the circuit the other way it is said to be reverse other way it is said to be reverse

biased. In this configuration biased. In this configuration electricity will not flowelectricity will not flow

Forward bias - all current, Forward bias - all current, almost no volts.almost no volts.

Reverse bias - all volts, Reverse bias - all volts, almost no current. almost no current.

Diodes are used for many Diodes are used for many reasons. One very important reasons. One very important

reason is to stop current flowing reason is to stop current flowing the wrong way through a circuit the wrong way through a circuit which can happen by placing a which can happen by placing a

battery in backwards.battery in backwards.

Another is to convert AC Voltage Another is to convert AC Voltage

to DC Voltageto DC Voltage

The picture below shows the The picture below shows the association between the schematic association between the schematic picture of a diode and the colour picture of a diode and the colour

coding of a diodecoding of a diodeNotice that the white band on the Notice that the white band on the diode is at the Cathode end of the diode is at the Cathode end of the

diodediode